Over the next few decades, coastal waters off of California, Oregon, and Washington are in danger of becoming acidic enough to harm the rich fisheries and diverse marine ecosystems there, according to a new study. Blame it on global warming's evil twin.

The process changing the seas' chemistry has been dubbed "ocean acidification." It refers to the impact that rising carbon dioxide levels in the atmosphere are having on seawater. CO2 levels are increasing as humans burn fossil fuel and change land-use patterns. The oceans absorb up to 26 percent of those emissions – a number that is expected to go up as the Arctic Ocean loses more of its summer sea-ice cover.

By 2050, the team conducting the study estimates, more than half the near-shore waters governed by the California Current system are likely to become so acidic throughout the year that many shell-building organisms will be unable to maintain their armor. That point could come within the next 20 to 30 years for some sea-floor habitats on the continental shelf, the researchers estimate.

While the team anticipated it would see marine conditions deteriorate with rising atmospheric CO2 levels, "I was really surprised to see how quickly some of these changes will be occurring," says Nicolas Gruber, a biogeochemist at the Swiss Institute of Technology in Zurich who led the team.